This present invention relates to encapsulated-lens reflective sheeting. More particularly, the invention relates to retroreflective sheeting containing an improved binder layer composition.
Encapsulated-lens retroreflective sheeting, also known as high intensity sheeting, is typically sold for use as traffic signs and other uses which need to be outdoor durable for a minimum of ten years. Therefore, highly durable and cost-effective materials must be used. Acrylic face films or cover films are desirable for this type of sheeting for several reasons, including their excellent durability, their receptiveness to highly durable screen printing inks, and their relatively low cost to produce.
For retroreflective performance, the sheeting also includes metallized glass microspheres which need to have an air gap above them, and the air gap is produced by encapsulation. In one embodiment, such sheeting may be prepared by the following procedure:
(1) Glass microspheres are embedded into a substrate which comprises a polyolefin such as a low-density polyethylene layer which is coated onto a polyester film. The substrate is heated to soften the polyolefin as the glass beads are brought into contact with and partially embedded into the polyolefin.
(2) The beaded polyolefin/polyester substrate is placed in a vacuum metalizer, and the surface containing the partially exposed glass beads is metallized with aluminum whereby a coating of aluminum is deposited on the exposed surface of the glass beads and the exposed surface of the polyolefin.
(3) A base sheet is prepared by coating a binder film comprising a thermoplastic polymer onto a release liner, and thereafter transferring the coated film by heat onto a support layer such as a 1 or 2-mil thick vinyl or polymethyl methacrylate layer which has been deposited on a casting substrate such as paper.
(4) The base sheet is then thermally bonded to the exposed aluminum coated surface of the glass beads by bringing the thermoplastic polymer surface into contact with the glass beads at an elevated temperature with mild pressure.
(5) The glass beads are removed from the polyolefin/polyester substrate by stripping the substrate from the base sheet. It is desired that the glass beads preferentially adhere to the thermoplastic polymer. The casting substrate (paper) may then be removed.
(6) The bead-containing base sheet with support layer is placed in contact with a preformed cover sheet, and the cover sheet is thermally laminated to the binder layer of the base sheet by embossing the laminate from the binder layer side with an embossing die having a predetermined pattern so that the embossing procedure results in the formation of a network of narrow intersecting bonds that extend between the cover sheet and the binder layer at the point of contact between the bonds and the cover sheet as the embossing pattern forces the cushion coat through the spaces between the beads and into contact with the cover sheet in selected areas. Thus, the cover sheet is thermally laminated to the cushion coat only in the bridge areas where the raised surface of an embossing die has been presses against the cushion coat and support layer. The glass beads in the bridge areas are surrounded by binder. At least a monomolecular air gap is formed between the beads and the cover sheet in the hermetically sealed areas formed by the network of intersecting bonds.
(7) An adhesive coated liner optionally can then be brought into contact with and laminated to the support layer which is beneath the binder layer.
The polymer materials which are selected for the binder film must comprise materials that are capable of retrieving the metalized glass beads from the polyolefin/polyester substrate, and materials which are also capable of firmly adhering to the cover film. These materials must also sufficiently flow when subjected to elevated temperature and pressure to perform these tasks, while at the same time they must be durable and dimensionally stable under conditions to which traffic signs may be exposed over extended periods of outdoor use.
It has been difficult to find individual thermoplastic resins for use as binder layers which are capable of both retrieving the metalized glass beads and forming permanent bonds to the cover film through heat lamination. Many resins which retrieve beads have poor adhesion to certain cover films such as acrylic films, and many resins which form permanent bonds to acrylic films through heat lamination are inefficient at retrieving beads. Some materials such as polycarbonates are capable of retrieving beads and adhering to acrylic films, but polycarbonates require a high processing temperature which is generally too high to be useful as a binder film.
Polyurethanes have been used as binder layers in retroreflective sheeting. For example, U.S. Pat. Nos. 4,897,136 (Bailey) and 5,064,272 (Bailey) disclose the use of a polyurethane resin as the binder film in encapsulated-lens retroreflective sheeting. The polyurethane binder film has a melt flow index of 750 or lower.
U.S. Pat. No. 5,882,771 (Klein et al) describes a conformable embossable enclosed lens retroreflective sheeting which contains a bead bond layer comprising an aminoplast crosslinked polymer containing urethane groups. Prior to cross linking, the polymer has a glass transition temperature of less than about 0xc2x0 C.
U.S. Pat. No. 5,378,520 (Nagaoka et al) relates to supported encapsulated-lens retroreflective sheeting. Binder materials for the glass microspheres are suggested as a binder materials having low glass transition temperatures (e.g., xe2x88x9220xc2x0 C. or lower). Thermoplastic polyurethanes are described as being especially useful as the binder material, and mixtures of polymeric materials also are suggested as useful binder materials. For example, a vinyl chloride/acetate copolymer is described as being mixed with a polyurethane polymer to provide a flexible binder material.
Encapsulated-lens retroreflective sheeting is described which contains an improved binder layer. The binder layer is obtained from a film-forming mixture which comprises:
(A) a vinyl chloride copolymer,
(B) a thermoplastic polyurethane, and
(C) an aminoplast resin.
Such binder layer exhibits improved adhesion to metallized beads and to a variety of cover films such as acrylic films.